The following description relates generally to a system for recovering carrier wave in digital broadcasting receiver, and more particularly to an apparatus for recovering carrier wave in digital broadcasting receiver and a method therefor capable of easily detecting and correcting frequency offsets of carrier wave without recourse to a pilot signal in a digital broadcasting receiver receiving a broadcasting signal of vestigial sideband (VSB) modulation system, thereby recovering the carrier wave.
Generally, a VSB transmission system of a Grand Alliance has been adopted as a standard for a transmission system of digital TV (e.g., SDTV/HDTV) in USA and Korea. The Grand Alliance VSB is a method for modulating one of sidebands generated by amplitude-modulating a signal. That is, in the Grand Alliance VSB, a signal is amplitude-modulated in order to generate two sidebands with a carrier wave as a center, one of sidebands is attenuated and a remained sideband is modulated. That is, the Grand Alliance VSB is one of the methods effectively using a band by obtaining a spectrum of one sideband in a baseband and shifting it to passband.
When a signal is modulated according to VSB, a Direct Current (DC) spectrum of the baseband is shifted to the passband and thus the DC spectrum is changed to a tone spectrum. This signal is commonly called as a pilot signal. A broadcasting station transmits the pilot signal to a receiver with a modulated signal for accurately demodulating the modulated signal at the receiver when a signal is modulated according to the VSB in the broadcasting station.
To be more specific, Annex D of the “ATSC Digital Television Standard” was published by the Advanced Television Systems Committee (ATSC) in September 1995 as its document A/53. This standard defined the broadcasting of digital television (DTV) signals within the United States of America and is referred to in this specification simply as “A/53”. A/53 specifies a vestigial-sideband amplitude-modulation signal in which the digital symbols are transmitted by eight-level modulation known as 8 VSB, which has −7, −5, −3, −1, +1, +3, +5 and +7 normalized modulation signal values.
In the VSB system, when a signal is amplitude-modulated, upper and lower sidebands are generated based on a carrier wave. At this point, when one of the two sidebands is greatly reduced, the other sideband is modulated. That is, only one sideband spectrum of a baseband signal is removed to a passband and then transmitted, such that the VSB modulation is more efficient in the use of bandwidth.
In the VSB modulation, if a direct current (DC) spectrum of a baseband signal is removed to a passband, the DC spectrum is converted to a tone spectrum. This signal is called a pilot signal. That is, when a broadcasting station performs the VSB modulation, the pilot signal is carried and transmitted together via air so that a receiver can correctly demodulate the signal.
FIG. 1 is a schematic block diagram of a digital broadcasting receiver receiving and recovering a broadcasting signal of the VSB modulation system.
First of all, an intermediate frequency (IF) signal inputted from an antenna may be converted to an IF passband signal, and the IF passband signal may be converted to an analog signal via an IF process unit 13 and inputted to an analog-to digital (A/D) converter 15. The IF process unit 13 typically may include a surface acoustic wave (SAW) filter for removing a high frequency component generated by a tuner and interferences of neighboring channels, and an automatic gain control (AGC) circuit for adjusting levels of input signals.
The A/D converter 15 may convert the analog passband signal outputted by the IF process unit 13 to a digital passband signal and output the digital passband signal to a phase splitter 17. For example, when the A/D converter 15 uses an oscillator, the analog signal may be converted to a digital signal having a fixed sampling frequency (25 MHz).
The phase splitter 17 may convert the digitalized passband signal to complex signals (I signal, Q signal), such that the digitalized signal is split to a real component passband signal and an imaginary component passband signal, each signal shifted by a 90-degree, and outputted to a carrier wave recovery unit 19. The carrier recovery unit 19 may convert the passband complex signals outputted from the phase splitter 17 to baseband complex signals and output the converted complex signals to a symbol timing recovery unit 21 for restoring symbol clocks.
The symbol timing recovery unit 21 serves to synchronize clocks of a transmitter terminal and clocks of a receiving terminal, and the symbol timing recovery unit 21 must ultimately operates in such a manner that the baseband signals or passband signals are sampled at an optimal point time-wise to minimize decision errors at an output side of a channel equalizer 27.
An output of the symbol timing recovery unit 21 passes a matched filter 23, where the matched filter 23 employs the square root raised cosine filter having a roll-off component, and an output signal of the matched filter 23 passes a direct current (DC) remover 25, from which a pilot signal is removed and the output signal is inputted to a channel equalizer 27.
The channel equalizer 27 removes the inter-symbol interference (ISI) contained in the signal that has passed the symbol timing recovery unit 21 and is outputted to a phase tracker 29. In other words, in a digital transmitting system, e.g., HDTV, bit detection errors may be generated at a receiving end by a distortion generated by a transmission signal that has passed a multi-pass channel, interference caused by NTSC signals, or a distortion caused by transmission/reception system. Particularly, propagation of signals via the multi-path channel generates ISI among the symbols to be a main cause of detection errors. The channel equalizer 27 removes the ISE among the symbols.
The phase tracker 29 corrects a residual phase of the carrier wave that is not completely removed by the carrier wave recovery unit 19 and outputs the corrected residual phase to a forward error correction unit 31. The forward error correction unit 31 corrects an error of phase-corrected signal and outputs the error to an audio/video (A/V) processor 33. The A/V processor 33 restores to the original signals the video and voice signals compression-processed by Moving Picture Experts Group-2 (MPEG-2) and Dolby AC-3 methods, where the video signal is transmitted to a monitor 37 to allow being viewed, and the voice signal is transmitted to a speaker 39 to allow being heard.
FIG. 2 illustrates exemplary views of the carrier wave recovery unit 19 and the symbol timing recovery unit 21 of FIG. 1.
Referring to FIG. 2, the carrier wave recovery unit 19 includes a Frequency and Phase Locked Loop (FPLL) system that simultaneously performs the frequency acquisition and tracking using pilot signals. Now, a process of recovering the carrier wave using the pilot signal will be described.
First of all, a passband signal is inputted to and passes through the phase splitter 17 in order to obtain a complex signal from the passband signal converted to the digital signal by the A/D converter. In doing so, the complex signal that has passed the phase separator 17 is converted to a baseband signal by the carrier wave recovery unit 19. When a first complex multiplier 19-1 of the carrier wave recovery unit 19 multiplies the passband complex signal that has passed the phase separator 17 by complex conjugate of fp.est, where fp.est is a frequency offset of a pilot signal estimated by fp, where fp is a pilot frequency, the pilot signal comes to be located at a position deviated from zero frequency as much as fp−fp.est, which is a frequency offset (Δfp). At this time, in order to reduce the influence by pattern jitter from the pilot signal having the frequency offset (Δfp), the pilot signal is made to pass the lowpass filter (LPF) 10-2, a carrier wave frequency error and a phase error are extracted by the known frequency/phase error detection means 19-3˜19-7 to correct the frequency offset of the basepass outputted from the first complex multiplier 19-1. The real component signal {Re (•)} outputted from the LPF 19-2 is directly inputted to the multiplier 19-4 or via a delayer 19-3, and the imaginary component signal {Imag (•)} is directly inputted to the multiplier 19-4. This type of construction is employed to broaden the frequency restoration scope. For example, a multiplexer (MUX) selects and outputs the real component signal delayed by the delayer 19-3 before the carrier wave frequency is restored, but after the carrier wave is recovered, the multiplexer (MUX) selects and outputs a real component signal that is not delayed.
Successively, the symbol timing recovery unit 21 restores the symbol timing from a signal converted to the baseband via the FPLL which is the carrier wave recovery system.
The symbol timing recovery unit 21 employs a typical Gardner timing error detector (TED), and in order to reduce the influence of pattern jitter by the shift of 8-level VSB symbols, only a bandpass filter 21-2 that takes only the upper sideband as the passband is made to pass, an error necessary for symbol timing restoration is extracted via the Gardner TED. The error thus obtained is lowpass-filtered by a loop filter 21-4 and generates a control signal of a re-sampler 21-1 by passing a numerically controlled oscillator (NCO). The re-sampler 21-1 converts the baseband digital signal sampled by the fixed frequency that was used during the A/D/conversion to a baseband digital signal sampled by a frequency corresponding to a frequency exactly twice the symbol frequency in response to a control signal from the NCO.
The symbol timing restored signal passes the matched filter 23 and is applied to the channel equalizer 27 via the DC remover 25.
In case of the carrier wave restoration system of FIG. 2, the carrier wave recovery entirely relies on the pilot signal, the frequency and the phase thereof, it is impossible to restore or recover the carrier wave, if a frequency corresponding to a position of the pilot signal under a selective frequency channel environment is greatly attenuated.
Furthermore, if a coherence bandwidth of a channel is much narrower than a signal band, the pilot signal is severely attenuated, and the lower sideband signals located about the pilot signal are not frequently attenuated. Under this circumstance, there occurs a problem in which the lower sideband signals operate as pattern jitters to bring forth an overall performance degradation of a receiver.